Determination of the most accurate position of a cylinder piston is a prerequisite for a whole range of technical applications. In particular, a precise piston position is required as the control variable for the automated activation of a piston cylinder unit. In addition, the determination of the position can contribute to safe operation since the inflow of the hydraulic medium, in particular the hydraulic fluid, can be precisely controlled in the extreme positions of the cylinder piston, and thus stopped in time.
Above all, precise determination of the position is important for the automatic control of piston cylinder units in construction machinery and lifting equipment. The piston cylinder unit actuates the working equipment of the construction machinery or lifting equipment in the usual manner. A sufficiently precise determination of the position of the piston cylinder unit increases the control quality and is therefore urgently needed.
The fact that the instantaneous position of the cylinder piston can be detected by mounted draw-wire potentiometers has already been established.
An alternative method for linear expansion measurements of the hydraulic cylinder operates according to a magnetostrictive principle. However, the piston rod must be bored axially into the cylinder for integration of the measuring elements. This is a very costly situation, especially for extremely long cylinders, that requires a great deal of effort using deep drilling technology. Likewise, guiding the cabling required for the measuring electronics used inside the cylinder represents a challenging task.
An additional method for determining the position of hydraulic and pneumatic cylinders is disclosed in DE 10 2005 029 494 A1. The piston cylinder arrangement discussed in this document has a sensor element that interacts with a magnetic probe element. The piston assumes the function of a plunger (probe element) whose position is determined by a path sensor coil (sensor element) and an evaluation circuit. The path sensor coil is located in the far side of the cylinder tube.
Another measurement principle is disclosed in DE 10 10 2011 088 A1. The resonance measurement procedure used here determines the instantaneous position of the piston based on the capacitance and inductance created by the piston cylinder unit. The measured inductance is crucial as the variable that changes in length. However, a disadvantage to the discussed measurement procedure is that sporadically occurring short circuiting effects between the piston and the outer surface, for example by contamination inside the piston cylinder unit, can distort the precision of measurement. This gives rise to a residual risk during the operation of such a cylinder unit.
The object of the present invention is thus to provide the practitioner with an alternative and secure measurement procedure for determining the position of the piston inside a piston cylinder unit.
This objective is achieved by a method according to the features of claim 1. Advantageous embodiments are presented in the dependent claims.
According to claim 1 a method is suggested for determining the piston position of a hydraulic or pneumatic piston cylinder unit. According to the invention, at least parts of the cylinder jacket and the cylinder piston form an electrically conducting path. The instantaneous piston position can be determined with sufficient precision from the inductive and/or resistive properties of the conducting path that is formed.
The invention makes use of the fact that the piston position influences the characteristics of the electrical conducting path. In particular, the length of the path varies with the piston position. The inductive and/or resistive properties of the electrical conducting path can be determined with the aid of suitable evaluation procedures. The values obtained allow a conclusion to be drawn regarding the actual instantaneous piston position.
It is especially advantageous when the cylinder piston is connected conductively with the cylinder jacket and the piston rod and cylinder jacket are insulated with respect to one another. The piston rod, piston, and cylinder jacket thereby form a mutual conducting path.
The conduction characteristics can advantageously be determined by means of an AC and/or DC evaluation procedure. To do this, the conducting path formed is excited with a DC or AC voltage signal, and the inductive or resistive behavior is determined by means of suitable evaluation procedures.
In evaluating the conductive characteristics based on DC or AC voltage signals, the advantage is that it can be carried out independent of the medium present in the cylinder chamber, namely air or a hydraulic medium. Aging of the oil, contamination, and possible temperature influences have no effect, or only extremely small effects, on the measured variables.
For an evaluation procedure based on AC voltages, it is advisable to design the frequency of the AC excitation voltage so that it is preferentially variable in order to assure frictionless operation of a plurality of piston cylinder units on the respective application (e.g., excavator). Shielding can be avoided by suitably selecting the frequency to lie in the low frequency region.
The type of evaluation procedure for determining the conductive characteristics is fundamentally arbitrary. A suitable evaluation procedure is to measure the external resonance of the piston cylinder unit or to measure a pulse response. The concept of an external resonance is intended to signify the resonance frequency of the piston cylinder unit that is to be measured as a function of position, which is obtained by the series connection of a capacitor with the piston cylinder unit. It is also possible to measure the ohmic resistance of the conduction path formed by means of a measuring bridge.
The suggested method has the advantage that it is applicable to any cylinder with an especially small amount of integration effort. In contrast to known procedures, the solution according to the invention is characterized by relatively low production and integration costs. In addition, the method according to the invention is not subject to piston cylinder unit size limitations. The suggested method can thus be implemented in almost any working cylinder.
The invention relates to a piston cylinder unit with a device for position determination according to the features of claim 5. According to the invention, the piston cylinder unit has a conducting connection between the piston and cylinder jacket, and in particular between the piston and the internal wall of the cylinder jacket. An evaluation unit is also provided that is connected directly or indirectly with the piston cylinder unit, and by means of which the instantaneous piston position can be determined as a function of the inductive and/or conducting path formed by the cylinder jacket and piston/-rod.
Ideally, the piston cylinder unit for carrying out the method according to the invention or an advantageous embodiment of the method according to the invention is appropriate. At the same time it is obvious that the piston cylinder unit must have suitable means for implementing the method. The advantages and properties of this piston cylinder unit obviously correspond to those of the method according to the invention or an advantageous embodiment of the method.
It is more reasonable if insulation is provided between the piston rod and the piston rod guide of the cylinder jacket, which enables insulated mounting of the piston rod with respect to the cylinder jacket. It is more reasonable if the existing piston rod seal is used to insulate the piston rod from the cylinder jacket.
In order to form a conducting path between the piston and cylinder jacket, at least one electrically conducting piston ring can be provided that passes fully around the circumference of the piston. It is also possible to use just a partial contact between the piston and cylinder jacket.
Alternatively, one or more sliding contacts can also be attached to the circumference of the piston or on the piston surface, which produce an electrically conducting contact between the piston and cylinder jacket.
The evaluation unit is advantageously electrically connected directly or indirectly to the piston and directly or indirectly to the cylinder jacket. Changing the piston position changes the electrical conducting path between the connectors of the evaluation unit. If the evaluation unit is connected in the region of the piston rod guide of the cylinder jacket, then the electrically conducting path that is formed lengthens as the piston is pushed into the cylinder jacket. The minimum conduction path is obtained as soon as the piston rod is completely withdrawn from the cylinder jacket. The piston stroke determines the minimum and maximum conduction paths.
It is obvious that the evaluation unit can also be connected to the opposite front surface of the cylinder jacket. The conductive characteristics have the opposite behavior.
The connection of the evaluation unit to the piston and the piston rod can either take place inside or outside the cylinder jacket. A fixed arrangement on the piston boss is especially advantageous. A flexible connection is also suitable, for example by means of a sliding contact or an alternative contacting technology.
The evaluation unit advantageously has a means for carrying out an evaluation procedure based on AC or DC voltages. In particular, the evaluation unit comprises one or more signal generators for producing and supplying an AC voltage or DC voltage signal in the conducting path that is formed.
The evaluation of the inductive and/or resistive properties of the conducting path that is formed can be achieved on the basis of different evaluation procedures. It is possible, for example, to measure the external resonance of the piston cylinder unit or to evaluate a pulse response. It is also possible that the evaluation unit has a measuring bridge by which the ohmic resistance of the connected conduction path can be determined.
The invention relates to a construction machine or lifting equipment having a piston cylinder unit according to the present invention, or to an advantageous embodiment of the invention. The advantages and properties of the construction machine or lifting equipment obviously correspond to those of the method according to the invention or the piston cylinder unit according to the invention, so that we will not repeat the description at this point.
Additional advantages and properties of the invention will be explained in greater detail with the aid of several exemplary embodiments.
FIGS. 1 to 3 show different embodiments of the piston cylinder unit 10 according to the invention with a device for determining the position. The design of the piston cylinder unit 10 is similar to a known piston cylinder unit. In particular the Unit 10 includes a tubular cylinder jacket 20, in whose cavity a piston 40 is mounted with a piston rod 30 that can be displaced linearly.